In wireless communication system, one or a multiple of common Downlink (DL) Reference Signals (RSs) may be used for coherent demodulation and channel measurement for each mobile terminal (also called a User Equipment (UE) in some systems) in a given cell. In case of multi-antenna transmission, an antenna is identified by a RS transmitted on that antenna. Each RS defines a so-called antenna port at a transmitter in a given cell. If multiple antennas use the same RS, they will belong to the same antenna port. RSs of different antenna ports should be orthogonal to each other in order to allow interference-free identification of each corresponding propagation channel coefficients at a receiver. The RSs are usually cell-specific to minimize interference between RSs belonging to different cells in a wireless communication system. The RSs are transmitted on exclusively reserved resources of a cell, such as on time and frequency Resource Elements (REs), codes, etc. To avoid interference, data is not transmitted on reserved resources allocated for RSs.
RSs are used for measurement of the radio channel and demodulation. For instance, a UE can determine Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) and Rank Indicator (RI) by measuring received RSs, and feedback measurement results including CQI/PMI/RI to a base station (such as a Node B or a eNB) for scheduling; or the UE can estimate the channel using the RSs, and use the estimated channel to demodulate data. The RSs used for measurement are usually common to all UEs in a cell, and cell-specific; the RSs used for demodulation can be common to all UEs or dedicated for a specific UE, and hence RSs used for demodulation can be cell-specific or UE-specific. RSs that are common to all UEs in a cell will be denoted Common RSs (CRSs).
In the Long Term Evolution (LTE) standard, measurement RSs and demodulation RSs share the same RSs, i.e. the same RSs are used for both measurement and demodulation, and are common to all UEs in cell-specific manner. In wireless communication systems with scheduling functionality, a base station first needs to know radio channel information for each UE, and then schedule the UEs based on the radio channel information. In order to obtain the radio channel information for each UE, the base station must transmit CRSs for all the UEs to measure the channel. Therefore CRSs are necessary in cellular wireless communication systems of this kind.
In the LTE standard, three types of cell-specific RSs are supported; defining one, two and four antenna ports (3GPP TS 36.211 v8.5.0). FIG. 1 illustrates how REs are used for transmission of RSs on each antenna port. It can be observed that resources for RSs of different antenna ports are orthogonal to each other through using different RE for each RS in Resource Blocks (RBs). Here a RB is defined as Nsymb consecutive OFDM symbols in the time domain and Nsc consecutive subcarriers in the frequency domain, but generally relates to radio resources in the frequency/time domain.
In order to be able to use CRSs properly in the DL, the key information which a UE needs is how many antenna ports that are used for DL transmission and the position of the RS on each antenna port. In the LTE standard, information about the number of antenna ports is embedded in a signal transmitted on a Physical Broadcast Channel (PBCH), and the position of the RS on each antenna port is associated with cell Identity (ID), which is conveyed in the Primary/Secondary Synchronization Signal (P/S-SS).
After successful cell search procedure, the UE will obtain time and frequency synchronization with a specific cell, as well as the cell ID for that cell. Based on the cell ID, the UE will know the RS on each antenna port in that cell. However, the UE will still not have information about the exact number of used antenna ports. Since this information is embedded in the PBCH signal, the UE has to make blind detection of that information, which means that it has to check all possible variants of the information and select the variant that is most probable conditioned on the received PBCH signal. The transmission structure for PBCH and P/S-SS according to the LTE standard is illustrated in FIG. 2 (note that the RSs are not shown in FIG. 2).
The first T OFDM symbols in each Sub-Frame (SF) are used for transmission of control information, such as Physical Downlink Control Channel (PDCCH), Physical Control Format Indicator Channel (PCFICH) and Physical Hybrid ARQ Indicator Channel (PHICH), where T=1, 2, 3, or 4. The area containing control information in a slot is called a control region, and the remaining resources in each slot belong to a non-control region. In the non-control region, all REs, except for the ones used for PBCH, P/S-SS and RSs, belong to the Physical Downlink Shared Channel (PDSCH) region.
In FIG. 2, P-SS and S-SS are transmitted on the last two OFDM symbols of slot0 (S0) and slot10 (S10), respectively, and are located at the central frequency part (centre six RBs) of the system bandwidth, while PBCH is transmitted in the central six RBs of slot1 (S1). In the RBs for transmission of PBCH information, data is mapped to time-frequency resources on the first four OFDM symbols as if maximum number of antenna ports is used (i.e. four in the LTE standard), i.e. as if all possible RSs in a cell are transmitted. In other words, the REs for all RSs in the first four OFDM symbols are reserved in a cell even if not all RSs are actually transmitted. An example is given in FIG. 3 for the illustration of PBCH resources corresponding to different number of antenna ports. For convenience, in the example, only the PBCH resources in one of the RBs used by the PBCH are shown.
In this way, the used resources for PBCH transmission are constant and independent of the number of used antenna ports, allowing for a single PBCH modulation and coding scheme, and consequently stable PBCH channel quality is independent of the number of used antenna ports. This is a key feature of PBCH channel that allows for blind detection of the embedded information relating to actually used antenna ports. That is, the information about the antenna port configuration is embedded into the PBCH by employing different Cyclic Redundancy Check (CRC) masks to indicate the number of antenna ports used.
In embedding the number of antenna ports used by a transmitter into PBCH, firstly, the entire PBCH transport block a0, a1, . . . , aA-1 is used to calculate the CRC parity bits p0, p1, . . . , pL-1, where A is the size of the transport block, i.e. the number of information bits, and L is the number of CRC parity bits which is set to 16 in the LTE standard. Secondly, according to the antenna port configuration of a specific cell, the CRC parity bits are scrambled by a sequence x0n, x1n, . . . x15n with length 16 corresponding to a certain number of antenna ports n, where n=1, 2 or 4. After scrambling, the masked CRC parity bits are c0, c1, . . . c15, where ci=(pi+xin)mod 2, i=0, 1, . . . , 15. Then, the masked CRC parity bits are attached to the transport block of PBCH to obtain the information bits as a0, a1, . . . aA-1, c0, c1, . . . , c15. The mapping relation between the three scrambling sequences and the number of antenna ports is shown in Table 1.
TABLE 1CRC mask for PBCH in LTENumber of antenna portsCRC mask sequence1<0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0>2<1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1>4<0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1>
Finally, a set of operations including channel coding, rate matching, modulation and resources mapping are performed on the information bits. If there is only one antenna port used, the modulation symbols are directly mapped to the reserved resources on antenna port 0; in the case of two antenna ports used, Space Frequency Block Coding (SFBC) is performed on the modulation symbols, and then the output of SFBC is mapped to the reserved resources on antenna port 0 and 1, respectively; and in the case of four antenna ports used, SFBC and Frequency Switching Transmit Diversity (FSTD) is performed on the modulation symbols, and then the output of SFBC+FSTD is mapped to the reserved resources on antenna port 0, 1, 2 and 3, respectively.
At the receiver side, corresponding inverse operations including resource de-mapping, decoding (SFBC or SFBC+FSTD), demodulation, channel decoding, CRC mask removal and CRC detection are performed by e.g. a UE accessing a cell. During the detection of PBCH, there are three hypothesises (one, two or four antenna ports) to be blindly detected by the receiver. Given one hypothesis, if the final CRC detection is correct, then the PBCH information bits and the information about the number of antenna ports will be obtained.
The LTE-Advanced (LTE-A) system is a wireless communication system intended to be an extension of the LTE system in which eight antenna ports defined by RSs may be supported to further increase system performance such as: peak data rate, cell average spectrum efficiency, etc (3GPP TR 36.913 0.1.1). However, in order to fulfill LTE-A backward compatibility requirements, it should be possible for a system to serve both LTE UEs and LTE-A UEs in a LTE-A cell, comprising up to eight RSs defining the same number of DL antenna ports, where LTE UEs are UEs configured according to the LTE system functionality and LTE-A UEs are UEs configured according to the LTE-A system functionality.